This paper investigates a number of numerical schemes applicable to the solution of the Boltzmann transport equation by means of a spherical-harmonics expansion (SHE). A new scheme is proposed that improves the solution at low energies, keeping the desired accuracy in the calculation of the mean quantities while saving a significant amount of CPU time. This is important in view of the applications of the method since the typical number of nodes to be used in the combined space-energy domain is in the range of 10/sup 4/-10/sup 5/.

The description of the optical-generation phenomena has been incorporated in the semiconductor-device analysis program HFIELDS, keeping the structural and geometrical flexibility of the original code. To this end a substantial effort has been devoted to the software implementation. The latter required the introduction of a number of optical windows and interleaved material layers through which a radiation with arbitrary spectrum, incidence angle, and polarization state enters the crystal. The corresponding generation rate at each node of the discretization grid is then evaluated by adding up the contribution of each monochromatic component of the impinging radiation. The code equipped with this new capability makes the description of realistic semiconductor optical sensors feasible. Black-body radiation has been incorporated into the code to allow for the simulation of solar cells. After a brief review of the underlying physics, the software implementation is described and a number of examples are illustrated.>